1. Field of the Invention
This invention relates to a fluorescent lamp and, more specifically, to a fluorescent lamp comprising an internal electrode provided inside a tubular glass bulb, an external electrode provided on an outer surface of the glass bulb, and a fluorescent layer formed on an inner surface of the glass bulb wherein the inside of the glass bulb is filled with a rare gas and the fluorescent layer is excited by ultraviolet light generated by discharge in the rare gas to emit light.
2. Background Art
FIG. 5 shows a configuration of this type of fluorescent lamp 90 of the prior art, which comprises a tubular glass bulb 91, an internal electrode 92, an external electrode 95, and a fluorescent layer 94. Inside the tubular glass bulb 91 which is sealed at both ends, the internal electrode 92 is provided at the center of the bulb 91 and a rare gas 93 is filled. The fluorescent layer 94 is formed on the inner surface of the glass bulb by suitable means such as coating. The external electrode 95 which also functions as a reflector is provided on the outer surface of the glass bulb 91 by affixing an aluminum foil, for example. Opening portions 94a and 95a each of which has an opening angle .alpha. from the center of the glass bulb 91 are formed in the fluorescent layer 94 and the external electrode 95 at the same position, respectively. Thus, the area of the formed fluorescent layer 94 is outwardly and indirectly covered with the area of the external electrode 95.
When voltage is applied between the internal electrode 92 and the external electrode 95 of the fluorescent lamp 90 configured as described above, discharge occurs between the both electrodes 92 and 95, whereby the fluorescent layer 94 is excited to emit the light which is radiated outward from the opening portions 94a and 95a. At this point, the light emitted from the fluorescent layer 94 to the glass bulb 91 is reflected by the external electrode 95, returns, passes through the fluorescent layer 94 and is radiated from the opening portions 94a and 95a.
However, in the fluorescent lamp 90 of the prior art described above, the amount of the emitted light is apt to decrease gradually with the passage of time after the start of lighting. Particularly, in the case of application where the lamp is turned on each time it is used and its use time is short, for example, when it is used as a writing light source for electrophotographic (PPC) printers or a reading light source for facsimiles, nonuniform density of the resulted matter occurs due to a sudden change in the amount of the light during a period of about 1 minute from the start of lighting, thereby deteriorating its printing quality.